The present invention relates generally to methods and apparatus for tubing conveyed perforating, and more specifically relates to methods and apparatus for perforating subsurface formations in response to pressure in a first tubing string and producing the formations through a second tubing string or through the casing. The present invention is particularly advantageous when multiple formations are desired to be perforated and produced together.
In oil and gas wells, it is often desirable to obtain production from multiple zones in a single well. In such wells, it may be desirable to perforate and produce from more than one formation within a single zone. Particular problems may be encountered when these formations are widely spaced. For example, it is known to perforate and produce formations located a thousand feet or more apart as a single zone. Difficulties presented in such situations include, for example, difficulties in obtaining reliable actuation of the detonating mechanism for the perforating guns, and particularly, doing so while maintaining a desired underbalance on the formations at the time of perforating.
Additionally, in many operations it is preferable to detonate the perforating guns through use of hydraulic pressure rather than by passing a mechanical detonating mechanism, such as a detonating bar, or "go devil," through the tool string. It is also preferable to utilize redundant firing mechanisms to optimize reliability of detonation of the perforating guns. Prior art techniques utilizing redundant firing mechanisms have required the pressurization of the annulus in the well adjacent the zone to be perforated. Such pressurization of the annulus can eliminate the ability to establish a desired pressure balance, typically an underbalance, between the perforated formation and the wellbore, to obtain optimal perforation of the formation.
Accordingly, the present invention provides a new method and apparatus for reliably perforating one or more formations in response to fluid pressure in a first tubing string while allowing the pressure differential between the formation and the wellbore to be established by a fluid column in a second tubing string or in the casing, through which the perforated formation may be flowed or produced. Additionally, the present invention facilitates the use of redundant firing mechanisms to maintain reliability of the perforating system. Additional formations, even when widely spaced, may be perforated without diminishing reliability of the system. Additionally, the first tubing string may be utilized to produce a second zone in the well.